Like the DM who lets the story of the campaign evolve from session to session, we can let our character's backstory evolve as we begin to know who they are. Our idea for their background may change. Until we start seeing our character going on adventures with the rest of the group, we don't really know that character. Then I'll spend the time to start filling in the details. As a player, I don't start filling out my backstory until I've played at least one adventure and reached 2nd level. You don't need to build out your character's backstories all at once. Describe it in one to five sentences instead of one to five pages. When developing the backstory for your character, keep that one-fifth spotlight in mind. We are, most of the time, one fifth of the group but our own character feels more important than that. As players, our own character is the hero of our journey and we don't often put other players' characters in the same spotlight. Our characters are the heroes of the story in our eyes. Consider how your character's mechanics can directly benefit the group. Building huge high-damage characters is fun but so is helping other characters do their thing best. When you have a choice for new spells or abilities, ask yourself which abilities help other characters and choose those as often as you dare.
Choose classes and abilities that support the other characters and you'll build a much stronger bond with the characters and players when you use them. DM David wrote a wonderful article about choosing character abilities that work well with other characters. The same is true for the mechanics you choose. The character doesn't have to like adventuring with others but they should be motivated to do so anyway. This doesn't mean you can't have a character where adventure is foreign or even undesired but they must still have the motivation to adventure with others. Nobility that would rather spend time alone with a good book in a royal palace often doesn't enjoy traveling through dungeons with a group of smelly adventurers. Often players come up with backstories that seem antithetical to the story of the campaign and traveling with a group. Why does my character want to travel with others while going on these adventures? Ask yourself this key question to help integrate your character into the group: Once you have a good idea what the themes of the campaign are and start to build your character around it, consider how you can build a character that fits in well with the group from both a story and mechanics standpoint. Help the DM steer the other players towards building characters that fit well with the story. Ask them how they plan to build their characters around the theme of the campaign as well. Once you have a handle on the theme of the campaign, spread that information around. Suggest they run a session zero if they're not already planning one and don't build your character fully until you're in that session zero. Ask them what skills will be most relevant. If your DM hasn't told you about the adventure or campaign yet, ask them. This leads to ham-fisted attempts to draw the character into the story and into the adventuring group. They plan out a bunch of levels ahead of time and never consider whether the themes of that character fit well with the themes of the campaign. They get excited about a particular class or a race and class combination and run with it. Many players dive right into character creation without considering the story or the theme of the campaign. Just as DMs can review the characters first while preparing their D&D games, the players can digest the theme of the adventure and campaign before building their characters both in story and in mechanics.